Polyorganosiloxane compounds, or “silicones,” can be synthesized in a wide variety of forms, and are utilized in numerous commercial applications. Silicone compounds are based on the repeating diorganosiloxane unit (—R2SiO—)n, where R is an organic radical and n denotes the number of units in the polymer chain. Each end of the linear chain is terminated with a functional or non-functional end group; the chain may also be “branched” so as to deviate from a strictly linear structure.
The physical properties of a particular silicone formulation depend on the length of the polymer chain, the nature of the organic functional groups bonded to the silicon atoms, and the terminal groups (more precisely, the alpha and omega groups) at each end of the chain. For example, the most common silicone compounds are based on the polydimethylsiloxane unit, —Si(CH3)2O—, which, due to the relatively small organic content of the chains, have a limited range of compatibility with organic compounds. By contrast, silicones containing aryl functional groups tend to exhibit properties more commonly associated with organic materials, and such silicones are generally miscible with a broader range of such materials.
Curing can be accomplished in a number of ways, but generally depends on the presence of reactive functional groups on the polymer chains that interact and bond with one another. In “condensation cure” reactions, a small molecule is eliminated when the two functional groups are joined. Typical condensation-cure reactions in silicone chemistry involve combination of silanol (—SiOH) functional groups with other such groups to produce an oxygen linkage with the elimination of water. “Addition cure” reactions result in no loss of species, and can involve, for example, hydrosilylation reactions between olefinic functional groups (such as vinyl) and hydrosiloxane groups (which have a silicon hydride moiety).
Variations on the traditional condensation-cure reaction include the “moisture-cure” approach, in which a precursor functional group is first hydrolyzed to form a reactive hydroxyl-bearing group, which then combines with a silanol group as discussed above. Suitable precursor compounds include acetoxy, alkoxy and ketoxime functional silanes, which form acid, alcohol and ketoxime byproducts, respectively, upon hydrolysis. Silanol-functional silicones and mixtures of silanol-functional silicones with silicones containing acetoxy, alkoxy or ketoxime groups are relatively stable so long as moisture is excluded; this is particularly true for silicone polymers having appreciable molecular weights. Obtaining useful reaction rates generally requires a catalyst such as a metal carboxylate compound.
Silanol groups also react with hydrosiloxane species to liberate hydrogen and produce the silicon-oxygen-silicon linkage characteristic of the condensation cures. Use of a metal salt catalyst (such as dibutyltindiacetate) is generally necessary to achieve useful reaction rates. Because it proceeds rapidly when catalyzed, this reaction is widely used for silicone coating formulations applied on a coating line to a web substrate.
Silicone rubber coatings have been adopted by some manufacturers of planographic printing plates. Planographic printing, as contrasted with letter-press and gravure printing, relies on plate constructions in which image and non-image areas lie substantially in the same plane. The plate is prepared by altering the affinities of different areas of the plate for ink. Depending on the type of plate imaging system employed, non-image plate areas become (or remain) oleophobic, or ink-repellent during printing, while image areas remain (or become) oleophilic, or ink-accepting. Ink applied to the plate surface (e.g., by a roller) will adhere to the oleophilic image areas but not the oleophobic non-image areas. The inked plate is then applied to the recording medium (in direct printing) or to an intermediate “blanket” cylinder which then transfers the image to the recording medium (in offset printing).
Manufacturers of planographic printing plates often employ silicone rubber compositions as plate coatings due to their low surface energies, which render them oleophobic and therefore suitable for “dry” or “waterless” plates. In contrast to the traditional “wet” plate, which requires application of a fountain or dampening solution to the plate prior to inking in order to prevent ink from adhering to and transferring from non-image areas, the non-image material of waterless plates is itself sufficiently ink-repellent that no fountain solution is necessary.
Silicone compositions used as coatings for planographic printing plates typically include two basic constituents: a primary polyorganosiloxane base-polymer component and a smaller cross-linking component. The base component is usually a linear, predominantly polydimethylsiloxane copolymer or terpolymer containing unsaturated groups (e.g., vinyl) or silanol groups as reactive centers for bonding with the cross-linking molecules. These groups are commonly situated at the chain termini, although it is possible to utilize a copolymer incorporating the reactive groups within the chain, or branched structures terminating with the reactive groups. It is also possible to combine linear difunctional polymers with copolymers and/or branch polymers.
The cross-linking component is generally a multifunctional, monomeric or oligomeric compound of low molecular weight, which is reacted with the first component to create connections among the chains thereof. The curing reaction generally requires some type of catalyst, either chemical or physical, to produce favorable kinetics. Platinum metal complexes (such as chloroplatinic acid) are often employed to facilitate addition cures, while metal salt catalysts (such as a dialkyltindicarboxylate) are frequently used in conjunction with condensation cures.
If the functional groups of the cross-linking component are situated at the chain termini, cross-linking molecules will form bridges among the base-polymer molecules (particularly if the latter have functional groups distributed along the chains). On the other hand, if the cross-linking component contains functional groups distributed along its length, each molecule will form numerous points of attachment with the base-polymer molecules. Typically, this type of cross-linking molecule is combined with base polymers having chain-terminal functional groups in order to maximize the number of different base-polymer molecules attached to each cross-linking chain.
In addition-cure systems, the hydrosilylation reaction provides fast-curing silicones that can be tailored to display strong bonding to hydroxyl-rich surfaces such as metal substrates. Silicone coating properties such as mechanical and adhesion performance depend on the extent of crosslinking and, therefore, the molar ratio of crosslinker to vinyl functional groups. The addition-curing chemistry involves at least three curing reactions indicated below at (1), (3) and (4). The primary reaction, (1), occurs between a vinyl-functional silicone polymer and a silicon-hydride (—SiH) functional group in the presence of a heavy metal catalyst compound, usually platinum or rhodium. Slower secondary reactions may occur, especially when silicones are formulated with an excess of crosslinker: the first of these is the catalyzed hydrolysis of silicon hydride groups to form —SiOH (reaction (2)). The newly formed silanol groups catalytically react with remaining —Si—H groups to form —Si—O—Si— bonds (reaction (3)). Finally, in the slowest condensation reaction, two silanol groups form an —Si—O—Si— linkage (reaction (4)). Reaction (1) is referred to as the “crosslinking” or “cure” reaction while reactions (2) through (4) are “post-cure” reactions.

When preparing silicone printing plates, the presence of a substrate interface increases the possibility of competing side-reactions. It is known that crosslinker silicon hydride groups and hydrolyzed silanol groups may react with substrate-borne hydroxyl groups. Silicone molecules attach to a substrate via two mechanisms: mechanical interlocking and chemical reaction. Mechanical interlocking occurs when silicone is applied to semi-porous substrates such as paper, but it cannot account exclusively for long-term stable anchorage of silicones to metal and polymer surfaces. The strength of silicone bonding depends mainly on chemical interactions between the silicone and a substrate. Metal surfaces exposed to air (or passivated metal) contain hydroxyl groups that interact with silicon hydride and silanol moieties forming a silyl ether linkage. FIG. 1 suggests a mechanism for the adhesion of silicone to metal substrates. These side reactions account for long-term adhesion of silicone to metal substrates, and illustrate the importance of using a large excess of crosslinker when coating addition-cured silicones on metals. Similarly, the adhesion of silicone to a polymer surface occurs when silicone crosslinker reacts not only with unreacted surface hydroxyls but also with available carboxyl groups.
A silicone coating composition is applied to a plate substrate using any of a variety of well-known coating techniques. The choice of technique is critical not only to the ultimate performance of the plate, but also to the efficiency and reliability of the overall platemaking process. Typical coating techniques include roll coating, reverse-roll coating, gravure coating, offset-gravure coating, slot coating, and wire-wound rod coating. The coating procedure must be rapid enough to achieve a satisfactory production rate, yet produce a highly uniform, smooth, level coating on the plate. Even small deviations in coating uniformity can adversely affect plate performance, since the planographic printing process depends strongly on coplanarity of image and non-image areas; in other words, the printing pattern reproduced by the plate must reflect the configuration of oleophilic and oleophobic areas impressed thereon, and remain uninfluenced by topological characteristics of the plate surface.
Blank dry plates are subjected to an imaging process that removes the silicone coating from image areas to reveal an oleophilic surface. Imaging can be accomplished in a number of ways. In traditional photosensitive plates, exposure of a photoresist material in the plate structure to actinic radiation alters the solubility or anchorage properties of the silicone. In typical commercial plates, exposure to light results either in firm anchorage of the silicone coating to the plate (in positive-working plates) or in destruction of the existing anchorage (in negative-working plates). Depending on the process chosen, the plate is first exposed to actinic radiation passing through a positive or negative rendition of the desired image that selectively blocks transmission of the radiation to the plate. After this exposure step, the plate is developed in chemical solvents that either anchor the exposed silicone or remove it to produce the final, imaged plate.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify photoexposure printing technologies, practitioners have developed electronic alternatives that store the “imagewise” pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. Current laser-based lithographic systems often rely on removal of an energy-absorbing layer from the lithographic plate to create an image. Exposure to laser radiation may, for example, cause ablation—i.e., catastrophic overheating—of the ablated layer, which de-anchors the silicone and facilitates its removal (along with remnants of the ablated “imaging” layer).
This imaging process and the rigors of commercial printing impart significant stress to the silicone rubber layer of a lithographic printing plate, and indeed, at the coating weights typically employed for economic reasons as well as to promote easy handling, such layers are inherently fragile and prone to scratching. Waterless printing plates fail when the silicone rubber loses ink repellency and non-image areas accept ink. Accordingly, there is a need for silicone coatings with improved robustness, leading to printing plates with enhanced durability and correspondingly longer run lengths.